| Abstract
| - The role played by the carbonyl group in the antenna system of a naturally occurring photochromic chromene,flindersine (FL), has been experimentally investigated and compared with that of a carbonyl group present ina structurally related unreactive heterocyclic compound, 6(5H)-phenanthridinone (PH). Through stationaryand time-resolved absorption and emission techniques, the excited-state relaxation dynamics after UV irradiationwere determined for FL and PH. The presence of a carbonyl group in both compounds entails the existenceof two close-lying, strongly coupled electronic excited states, having n,π* and π,π* character, respectively.Their coupling can be modulated by a careful choice of the solvent proticity and temperature. Moreover, inthe case of strong coupling between the n,π* and π,π* states, we have proved that the relaxation dynamicscan involve transitions in which the upper of the coupled states acts as an intermediate for radiationlessdecay, bypassing the lowest emissive state, whereby the fluorescence quantum yield becomes a function ofthe excitation wavelength.
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